Page 14 - A bird’s-eye view of recreation - Rogier Pouwels

A bird's-eye view of recreation
breed or forage in groups (Stillman et al. 2007, Bennet et al. 2011) or on the ground (Kangas et al. 2010, Thompson 2015) and species that are large (Weston et al. 2012), in decline (Møller 2008) or are not able to habituate to human presence (Klein et al. 1995, Miller et al. 2001). For example, Miller et al. (1998) showed that in North America densities of most species (Western Meadowlark, Sturnella neglecta, Pygmy Nuthatch, Sitta pygmaea, Western Wood-pewee, Contopus sordidulus, Chipping Sparrow, Spizella passerina and Solitary Vireo, Vireo solitarius) increased further away from trails, while densities of American Robin (Turdus migratorius) were higher near the trails. Also, data on Black-billed Magpies (Pica pica) and House Finches (Carpodacus mexicanus) indicated that densities were positively affected by the presence of trails, while Vesper Sparrow, (Pooecetes gramineus), Grasshopper Sparrow, (Ammodramus savannarum) and Townsend's Solitaire, (Myadestes townsendi) were more abundant in control sites than in sites near trails (Miller et al. 1998).
A second explanation for ambiguity lies in the complexity of the disturbance mechanisms. Disturbance will only occur at locations where a visitor and a bird are present at the same time (Pickering 2010). A disturbance event will result in physiological and behavioural responses by the individual bird (Le Corre et al. 2009), but this disturbance will not necessarily have an impact on the viability of the total bird population. A disturbance response by an alarmed bird involves the use of extra energy and time flying away, energy and time that is needed for vital activities like parental warming and feeding (Yalden and Yalden 1990, Verhulst et al. 2001, Goss- Custard et al. 2006). When visitor numbers increase, the sum of all disturbance events may lead to a critical reduction in available energy and a lower fitness of individual birds. As a consequence, the young in the nest could lose weight, increasing the probability of them dying. The larger the number of birds disturbed in an area, the higher the chance of an effect on densities, reproduction success or survival. Where high visitor numbers are spread over a large area, disturbance has been shown to have impacts on population size and population viability (Le Corre et al. 2009, Steven et al. 2011) (Fig. 1).
Scientific evidence is not evenly distributed over the different stages of this cascade of impacts (Buckley 2013). Most research on the impact of visitors on birds examines behavioural responses by individual birds; only a few studies look at population-level impacts (Blanc et al. 2006, Sutherland et al. 2006, Le Corre et al. 2009, Steven et al. 2011). Research into behavioural impacts can be conducted on individual birds, while research into population viability can only be conducted at larger spatial scales. However,
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